Polystyrene-supported triazoles for metal ions extraction: Synthesis and evaluation

Reactive & Functional Polymers 74 (2014) 37–45

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Polystyrene-supported triazoles for metal ions extraction: Synthesis and evaluation Abid Ouerghui b,c,⇑, Hichem Elamari a,b, Saadia Ghammouri d, Riadh Slimi a,b, Faouzi Meganem b, Christian Girard a,⇑ a Unité de Pharmacologie Chimique, Génétique & Imagerie, CNRS UMR8151, INSERM U1022, Ecole Nationale Supérieure de Chimie de Paris (Chimie ParisTech), Paris Sciences & Lettres (PSL Research University), 11 rue Pierre & Marie Curie, Paris 75005, France b Laboratoire de Chimie Organique et Applications, Faculté des Sciences de Bizerte, Université de Carthage, 7021 Jarzouna, Bizerte, Tunisia c Département de Chimie, Faculté des Sciences de Gafsa, Sidi Ahmed Zarroug, 2112 Gafsa, Tunisia d Unité de Biologie Moléculaire et Génétique (B.M.G), Faculté des Sciences de Gafsa, Université de Gafsa, 2100 Gafsa, Tunisia

a r t i c l e

i n f o

Article history: Received 29 April 2013 Received in revised form 17 August 2013 Accepted 5 October 2013 Available online 24 October 2013 Keywords: Polystyrene Clays Triazole Click chemistry Metal extraction

a b s t r a c t In order to prepare substituted polymers bearing functional groups to chelate metals for their application in extraction and/or depollution applications, Merrifield polymer was transformed into the known azidomethyl polystyrene. Click-chemistry based on copper (I)-catalyzed Huisgen’s reaction was then used to form polymer-grafted 1,4-triazoles using a variety of synthesized substituted alkynes. These polymer-supported triazoles were then used to extract metals (Cd, Fe, Mg, Ni and Co) from aqueous solutions. A comparative study of metal extractions by these supported triazoles was made between the starting azidomethyl polystyrene and two natural clays taken from the Gafsa area (South-West Tunisia). Raw and purified clays from two Gafsa sites were found to extract metals quite well with almost no selectivity, except for lower fixations of cadmium and magnesium. The synthesized polymers were found to extract all metals with lower efficiencies than the clays. However, one of the polymer-supported triazole was found to extract selectively cadmium with a high efficiency, reaching the levels of the natural clays. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Solid–liquid extraction of metals and contaminants is a longtime known field that has gained interest during the past year, thanks to the greener approaches to chemistry and better control of industrial effluents for obvious ecological reasons. Many substrates can be used for the removal of pollutants that range from natural clays and other minerals, human made zeolites and ceramics, to functionalized polymers. Ion-exchange polymers, as an example, are known for a long time [1–3]. The polymers can be more hydrophilic [4–7] or hydrophobic [8,9] in nature, depending of the desired application [10–14]. Polymers substituted by chelating entities, ionic or not, have found new applications in supported catalysis as well as for the depollution, or purification, by their ability to interact with different metals [15–20]. ⇑ Corresponding authors. Address: Laboratoire de Chimie Organique et Applications, Faculté des Sciences de Bizerte, Université de Carthage, 7021 Jarzouna, Bizerte, Tunisia. Tel.: +216 995 241 240 (A. Ouerghui). Tel.: +33 144 276 748; fax: +33 144 276 496 (C. Girard). E-mail addresses: [email protected] (A. Ouerghui), christian-girard@ chimie-paristech.fr (C. Girard). 1381-5148/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.reactfunctpolym.2013.10.007

Based on the experience of both our laboratories, we became interested by the comparison between engineered polymers and natural clays for such purposes [21–31]. The preparation of the desired polymers was based on the concept of ‘‘click-chemistry’’ using especially the copper(I)-catalyzed Huisgen’s cycloaddition (‘‘copper (I)-catalyzed azide/alkyne cycloaddition’’ or CuAAC) in this field [32–37]. The basic idea was to graft on an azided polymer a set of functionalized alkynes using such a reaction [38–41]. The formed 1,4-triazole units in this approach (‘‘triazole design’’), as well as the appendages on this heterocycle (‘‘pendant design’’), or both (‘‘integrated design’’), can participate in the chelation of metals; alone or with other subunits on the polymer network (Fig. 1) [42,43]. We thus present in this article the preparation of some polymer-supported triazoles starting from Merrifield resin (chloromethylated polystyrene), followed by its transformation in azido derivative and CuAAC with selected alkynes. These polymers were then used to extract metals salts (Mg2+, Fe3+, Co2+, Ni2+ and Cd2+) from aqueous solutions. Extraction results were compared to those obtained with natural clays from the Gafsa area (Tunisia).

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A. Ouerghui et al. / Reactive & Functional Polymers 74 (2014) 37–45 pendant

triazole

integrated

R

R

R MX

R

R

n

N3 copper (I)

N N N

MX

MX

N N N

N n N N

N MX N N n

= polymer

and / or

and / or

Fig. 1. General scheme for the preparation of triazolic polymers and possible chelation ways (pendant, triazole and integrated designs).

2. Experimental

NaN3

Cl

2.1. Materials and apparatus 2.1.1. Materials Merrifield polymer (chloromethylated poly-styrene-co-divinylbenzene (1%), 100–200 mesh, 2.1 mmol Cl g1) was purchased from Fluka (France), washed alternatively three times with methanol and methylene chloride (10 mL g1) and dried before use. Acyl chlorides (99%), propiolic acid (95%), propargyl amine (98%), substituted anilines, N,N-dimethyl-aminopyridine (DMAP) (99%), N,N0 -dicyclohexylcarbodiimide (DCC) (99%), triethyl and diisopropylethyl (DIPEA) amines (99%), phenylacetylene (99%), methyl propiolate (97%), sodium azide (99%) and copper (I) iodide (98%) were provided by Sigma–Aldrich (France) and used without further purification. Methylene chloride and dimethylsulfoxide (DMSO) were bought from SDS-Carlo-Erba (France) and used as received. Tetrahydrofuranne (THF) was from the same source but distilled from benzopheno-ne ketyl under nitrogen prior to use. CdCl2H2O (98%), Fe(NO3)39H2O (99%), NiCl26H2O (98%) and CoCl26H2O (98%) were produced by Fluka (France) and MgCl26H2O (99%) by Panreac Quimica SA (Barcelona, Spain), and used as such. Samples of clays from Gafsa were collected on two sites (Jebal Es Sath and Jebal Sehib) and used raw or purified accordingly to the method described in the experimental part.

N3

DMSO 60 C, 48 h Merrifield's polymer

A

Scheme 1. Preparation of azido methyl polystyrene (A) from Merrifield resin.

Et3N cat. DMAP

O R1

+

H2N

Cl

CH2Cl2 0 C to R.T.

acyl chlorides

2.2. Organic syntheses 2.2.1. Syntheses of azidomethyl polystyrene (A) and substituted alkynes (1a–d, 2a–d) Organic syntheses (Schemes 1–3, Section 3.1) were conducted under Nitrogen, otherwise stated, using standard methods and procedures. Characterizations were made as described below. Melting points (mp) were measured on a Kofler apparatus after calibration around the observed fusion of the product. Infrared

N H

propargylamine

1a-d

R1 = 1b, 67 %

1a, 76 %

1c, 66 %

1d, 79 %

Scheme 2. Synthesis of alkynes 1a–d from acyl chlorides and propargylami-ne.

R2

DCC

O NH2

anilines

+

HO

O R2

CH2Cl2 0 C to R.T.

N H

propiolic acid MeO

2a-d O

O

R2 =

2.1.2. Apparatus Characterizations were made as described below. Melting points (mp) were measured on a ‘‘Original Kofler’’ apparatus (43 cm  14 cm, 50–260 °C, 100 W, Fisher Scientific Bioblock, France). Infrared analysis using the attenuated total reflectance technique (ATR/FTIR) was made on a Nicolet FTIR 200 spectrophotometer (Thermo Scientific, France). Nuclear magnetic resonance spectroscopy (NMR) was recorded on a Bruker Avance 300 WB spectrometer (Bruker, France) at 300 MHz for the proton spectra (1H) and 75.5 MHz for the carbon spectra (13C). Differential scanning calorimetry (DSC) was performed on a Setaram DSC 131 (France). X-ray diffraction (XRD) was recorded on a Thermo Scientific ARL 9900 XRF (France). The amount of remaining metal ions in solution was evaluated by atomic absorption spectroscopy (AAS) analysis on a Perkin–Elmer AAnalyst 200 (France). Elemental analysis of N was performed by using Perkin Elmer Analyzer CHN Series II 2400 (France). The DTA and TGA were obtained with a Setaram SETSY-1750 (France).

O R1

O

O MeO 2a, 70 %

2b, 70 %

O 2c, 69 %

O

O 2d, 67 %

Scheme 3. Synthesis of alkynes 2a–d from anilines and propiolic acid.

analysis using the attenuated total reflectance technique (ATR/ FTIR) was recorded using pure samples between 4000 and 400 cm1. Only the main and relevant absorption bands are indicated as vibrations (m) and angular deformations (d). Nuclear magnetic resonance spectroscopy (NMR) was recorded in deuteriochloroform (CDCl3) or deuterated dimethylsulfoxide (DMSO-d6) with calibration onto the residual 1H signal of the solvent. Chemical shifts (d) are indicated after calibration on the residual undeuterated solvent peak in part per million (ppm) and as follow: s (singlet), d (doublet), t (triplet), sex (sextuplet), m (multiplet). Coupling constants (J) are given in Hz. 2.2.1.1. Synthesis of azidomethyl polystyrene (A). The polymer A was prepared according to literature [44]. Merrifield polymer (2.1 mmol Cl g1, 10 g, 21 mmol Cl) was suspended in 100 ml DMSO and 6.82 g NaN3 (105 mmol, 5 eq.) were added. The reaction mixture was slowly magnetically stirred at 60 °C for 48 h. The cooled mixture was filtered on sintered glass and washed 3 times with DMSO (60 mL) and finally dried under vacuum at 60 °C for 48 h. The isolated polymer isolated weighted 10.097 g. The substitution yield, calculated by weight increase and ATR/FTIR (data not shown) was 50%. Substitution was calculated with the results

A. Ouerghui et al. / Reactive & Functional Polymers 74 (2014) 37–45

elemental analysis (8.234% N) and found to be 5.88 mmol N, for a final substitution of 1.96 mmol N3 g1. ATR/FTIR: m = 3083, 2931, 2094 (mN3), 766, 706 cm1. 2.2.1.2. Synthesis of propargyl amine derivatives 1a to 1d (general method). A stirred solution 0.65 mL of propargyl amine (0.56 g, 10 mmol), 1.4 mL of triethyl amine (1.0 g, 10 mol, 1 eq.) and 24 mg of DMAP (0.2 mmol, 2%mol) in 20 mL of CH2Cl2 was cooled to 0 °C. A solution of the acyl chloride (10 mmol) in 20 mL of CH2Cl2 was then added dropwise. The reaction mixture was then stirred at 0 °C for 0.5 h at over-night at room temperature. The mixture was then extracted with 0.1 M aqueous solutions of HCl (2  10 mL) and NaOH (2  10 mL), washed with water (3  20 mL), dried over MgSO4, filtrated and the solvent evaporated under vacuum. The resulting amides 1a–d were sufficiently pure to be used without further purification. N-prop-2-ynylacetamide (1a): C5H7NO. MW = 97.12 g mol1. Yield = 76%. mp = 82 °C. ATR/FTIR: m = 3231, 2928, 2852, 2115, 1641 cm1. 1H NMR (300 MHz, CDCl3, d): 2.05 (s, 3H), 2.27 (t, J = 2.57, 1H), 4.09 (dd, J = 5.25, 2.57 Hz, 2H), 5.72 (s, 1H) ppm. 13C NMR (75.5 MHz, CDCl3, d): 22.94, 29.19, 71.38, 79.67, 170.11 ppm. N-prop-2-ynylbutyramide (1b): C7H11NO. MW = 125.17 g mol1. Yield = 67%. mp = 27 °C. ATR/FTIR: m = 3294, 3049, 2928, 2117, 1641 cm1. 1H NMR (300 MHz, CDCl3, d): 0.98 (t, J = 7.37 Hz, 3H), 1.72 (sex, 2H), 2.22 (m, 3H), 4.08 (dd, J = 5.23, 2.56 Hz, 2H), 5.72 (s, 1H) ppm. 13C NMR (75.5 MHz, CDCl3, d): 13.74, 19.03, 29.11, 38.33, 71.44, 79.75, 172.75 ppm. N-prop-2-ynylcaprylamide (1c): C11H19NO. MW = 181.28 g mol1. Yield = 66%. mp = 64 °C. ATR/FTIR: m = 3294, 2958, 2920, 2118, 1642 cm1. 1H NMR (300 MHz, CDCl3, d): 0.91 (t, J = 6.85 Hz, 3H), 1,33 (m, 8H), 1.66 (m, 2H), 2.23 (m, 3H), 5.95 (s, 1H), 4.07 (dd, J = 5.20, 2.52 Hz, 2H) ppm. 13C NMR (75.5 MHz, CDCl3, d): 13.91, 22.37, 25.25, 29.12, 31.41, 36.42, 71.44, 79.72, 172.86 ppm. N-prop-2-ynylbenzamide (1d): C10H9NO. MW = 159.19 g mol1. Yield = 79%. mp = 106 °C. ATR/FTIR: m = 3286, 3049, 2930, 2123, 1641, 1599, 1537 cm1. 1H NMR (300 MHz, CDCl3, d): 2.33 (t, J = 2.56 Hz, 1H), 4.31 (dd, J = 2.54, 5.17 Hz, 2H), 6.31 (s, 1H), 7.56 (m, 1H), 7.48 (m, 2H), 7.82 (m, 2H) ppm. 13C NMR (75.5 MHz, CDCl3, d): 29.80, 71.80, 79.61, 127.13, 128.62, 131.80, 133.79, 167.29 ppm. 2.2.1.3. Synthesis of propiolic acid derivatives 2a to 2d (general method). To a stirred cold solution (0 °C) of 10 mmol of the aniline derivative was added dropwise a solution of 0.62 mL of propiolic acid (0.71 g, 10 mmol, 1 eq.) in 20 mL of CH2Cl2, followed by a solution of 2.58 g of DCC (12.5 mmol, 1.25 eq.) in 20 mL of CH2Cl2. The reaction mixture was then stirred at 0 °C for 0.5 h and over-night at room temperature. The mixture was then filtered through Celite onto sintered glass, the Celite washed with CH2Cl2 (20 mL). The filtrate was then extracted with 0.1 M aqueous solutions of HCl (2  10 mL) and NaOH (2  10 mL), washed with water (3  20 mL), dried over MgSO4, filtrated and the solvent evaporated under vacuum. The resulting amides 2a–d were sufficiently pure to be used without further purification. Otherwise, the compounds are purified on silica gel using heptane:ethyl acetate (1:1). N-phenylpropiolamide (2a): C9H7NO. MW = 145.16 g mol1. Yield = 70%. mp = 111 °C. ATR/FTIR: m = 3342, 3021, 2966, 2110, 1746, 1638, 1591 cm1. 1H NMR (300 MHz, DMSO-d6, d): 4.42 (s, 1H), 7.12 (m, 1H), 7.35 (m, 1H), 7.63 (d, J = 8.22 Hz, 1H), 10.84 (s, 1H) ppm. 13C NMR (75.5 MHz, DMSO-d6, d): 77.50, 78.91, 120.25, 124.72, 129.30, 138.68, 150.16 ppm. N-(3,4-dimethoxyphenyl)propiolamide (2b): C11H11NO3. MW = 205.22 g mol1. Yield = 70%. mp = 135 °C. ATR/FTIR: m = 3286, 3070, 2920, 2115, 1642, 1576, 1558 cm1. 1H NMR

39

(300 MHz, DMSO-d6, d): 2.94 (s, 1H), 3.89 (s, 3H), 3.91 (s, 3H), 6.84 (d, J = 8,64 Hz, 1H), 6.97 (dd, J = 8.64, 2.46 Hz, 1H), 7.31 (d, J = 2.46 Hz, 1H), 7.71 (s, 1H) ppm. 13C NMR (75.5 MHz, DMSO-d6, d): 55.99, 56.11, 76.62, 77.69, 105.00, 111.31, 112.15, 130.52, 146.53, 149.12, 149.56 ppm. N-(2,3-dihydrobenzo[1,4]dioxin-6-yl)propiolamide (2c): C11H9NO3. MW = 203.20 g mol1. Yield = 69%. mp = 134 °C. ATR/ FTIR: m = 3262, 2959, 2920, 2109, 1641, 1605, 1547 cm1. 1H NMR (300 MHz, DMSO-d6, d): 4.23 (s, 4H), 4.37 (s, 1H), 7.22 (s, 1H), 7.04 (d, J = 8.74 Hz, 1H), 6.82 (d, J = 8.72 Hz, 1H), 10.66 (s, 1H) ppm. 13C NMR (75.5 MHz, DMSO-d6, d): 64.45, 64.66, 77.29, 78.97, 109.35, 113.44, 117.36, 132.32, 140.53, 143.40, 149.73 ppm. N-(6,7,9,10,12,13,15,16-octahydro-5,8,11,14,17-pentaoxabenzocyclopentadecen-2-yl)propiolamide (2d): C17H21NO6. MW = 335.36 g.mol1. Yield = 67%. mp = 148 °C. ATR/FTIR: m = 3377, 3282, 2857, 2094, 1654, 1632, 1517 cm1. 1H NMR (300 MHz, CDCl3, d): 2.95 (s, 1H), 3.78 (s, 8H), 3.92 (m, 4H), 4.14 (m, 4H), 7.31 (d, J = 2.32 Hz, 1H), 6.95 (dd, J = 8.63, 2.33 Hz, 1H), 6.82 (d, J = 8.63 Hz, 1H), 7.92 (s, 1H) ppm. 13C NMR (75.5 MHz, CDCl3, d): 68.75, 69.31, 69.50, 70.35, 70.39, 70.84, 74.46, 76.73, 107.19, 112.95, 114.36, 131.45, 146.15, 149.05, 149.95 ppm. 2.2.2. Synthesis of polymer-supported triazoles 3a–e and 4a–e (general method) Coupling reactions onto the polymer using CuAAC were conducted accordingly to the general procedure indicated below in round bottom flasks equipped with a reflux condenser. Polymers 3e and 4e were synthesized using commercially available phenylacetylene (1e) and methyl propiolate (2e), respectively (Scheme 4, Section 3.2). To a suspension of 0.3 g of azidomethyl polystyrene A (1.96 mmol g1, 0.6 mmol) in THF (10 mL) was added 3 mmol (5 eq.) of the alkyne (1a–e, 2a–e), 1 ml of DIPEA (0.7 g, 6 mmol, 20 eq.) and 2.4 mg of copper (I) iodide (0.013 mmol, 2 mol% based on N3). The suspension was slowly stirred at room temperature until the complete disappearance of the IR band of the azide of the polymer (2094 cm1, ca. 24 h), the transformation being considered as quantitative. The resulting polymer was filtered on sintered glass and washed with MeOH, CH2Cl2 and pyridine (5  5 mL) and finally dried under vacuum. [4-(acetamidomethyl)triazol-1-yl]methyl polystyrene (3a): 1.67 mmol triazole g1. ATR/FTIR: m = 3276, 3047, 2933, 1662, 1548, 1519, 1450 cm1. [4-(butyramidomethyl)triazol-1-yl]methyl polystyrene (3b): 1.60 mmol triazole g1. ATR/FTIR: m = 3297, 3052, 2937, 1666, 1519, 1458 cm1. [4-(caprylamidomethyl)triazol-1-yl]methyl polystyrene (3c): 1.47 mmol triazole g1. ATR/FTIR: m = 3303, 3038, 2931, 1660, 1512, 1447 cm1. [4-(benzamidomethyl)triazol-1-yl]methyl polystyrene (3d): 1.52 mmol triazole g1. ATR/FTIR: m = 3351, 3032, 2931, 1660, 1527, 1447 cm1. (4-phenyltriazol-1-yl)methyl polystyrene (3e): 1.66 mmol triazole g1. ATR/FTIR: m = 3038, 2931, 1500, 1453, 1696 cm1. {4-[N-(phenyl)aminocarbonyl]triazol-1-yl}methylpolystyrene (4a): 1.55 mmol triazole g1. ATR/FTIR: m = 3403, 3031, 2933, 1686, 1604, 1567, 1516, 1438 cm1. {4-[N-(3,4-dimethoxyphenyl)aminocarbonyl]triazol-1-yl}me-thyl polystyrene (4b): 1.42 mmol triazole g1. ATR/FTIR: m = 3390, 3031, 2932, 1680, 1608, 1564, 1520, 1450 cm1. {4-[N-(2,3-dihydrobenzo[1,4]dioxin-6-yl)aminocarbonyl]tri-azol1-yl}methyl polystyrene (4c): 1.42 mmol triazole g1. ATR/FTIR: m = 3296, 3040, 2939, 1674, 1611, 1573, 1516, 1423 cm1. {4-[N-(6,7,9,10,12,13,15,16-octahydro-5,8,11,14,17-penta-oxabenzocyclopentadecen-2-yl)aminocarbonyl]triazol-1-yl}me-thyl polystyrene (4d): 1.20 mmol triazole g1. ATR/FTIR: m = 3394, 3041, 2924, 1686, 1604, 1516, 1460 cm1.

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A. Ouerghui et al. / Reactive & Functional Polymers 74 (2014) 37–45

R

CuI (3 mol%) +

N3

R

N

DIPEA, THF R.T., 24 h A

N

N 3a-e 4a-e

1a-d 1e, R = Ph 2a-d 2e, R = CO2Me

R= O N H 3a O N H

O

2 N H

6 N H

3b

3c

MeO

O

MeO

4a

O

O

N H

N H 3d

O

O

O

O

4c

4b

O

O

O

O

N H

O

3e

O

MeO

N H

4e

4d

Scheme 4. Cu(I)-catalyzed Huisgen’s reaction of azidomethyl polystyrene (A) with various alkynes (1a–e/2a–e) to obtain the triazolic polymers 3a–e and 4a–e.

2.3. Characterizations by ATR/FTIR, DSC and DTA–TGA of polymersupported triazoles 3a–e and 4a–e and azidomethyl polystyrene (A) Infrared spectra (ATR/FTIR) were recorded as previously decribed and are presented in Figs. 2, 3 and 6 (Section 3.2). Differential scanning calorimetry (DSC) was performed on 2 mg samples placed in 30 lL aluminum pans, and the DSC recorded under argon between 25 and 700 °C at the speed of 10 °C min1. Results appear in Fig. 4 and Table 1 (Section 3.2). The DTA and TGA recordings were conducted on a 10.73 mg sample at the speed of 5 °C min1, are presented in Fig. 5 (Section 3.2)

(4e)

120 (4d)

TRANSMITTACE

[4-(methoxycarbonyl)triazol-1-yl]methyl polystyrene (4e): 1.71 mmol triazole g1. ATR/FTIR: m = 3447, 3039, 2932, 1730, 1497, 1441 cm1.

(4c)

100

(4b)

(4a)

(A)

80 4000

3000

2000

1000 -1

Wave number (cm )

2.4. Natural clays: harvesting and purification

Fig. 3. ATR/FTIR spectra of triazolic polymers 4a–e and of the starting azidomethyl polystyrene (A).

EXO

The natural clays were harvested at 40 cm below the surface on two sites: Jebal Es Sath and Jebal Sehib (as shown in Fig. 6, Section 3.3). The composition of the raw clays was determined using a X-ray fluorescence (XRF) coupled with diffraction (XRD) on a Thermo Scientific ARL 9900 XRF series workstation. The raw clay was grounded and sieved (100 lm) and used as such for the

4e 4d 4c 4b 4a

Heat Flow ( µ w)

130 (3e)

3e 3d 3c

TRANSMITTANCE

0

(3d)

120

(3c)

110

3b 3a A

(3b)

(3a)

0

200

400

600

100 (A)

Fig. 4. DSC diagrams of azidomethyl polystyrene (A) and the triazolic polymers 3a– e and 4a–e.

90 4000

3000

2000

1000 -1

wave number (cm ) Fig. 2. ATR/FTIR spectra of triazolic polymers 3a–e and of the starting azidomethyl polystyrene (A).

metals extraction or for the purification procedure (sodium exchange). The exchange, to obtain the purified clay, was done by placing 20 g of the sieved raw clay in suspension in 400 mL of a

41

A. Ouerghui et al. / Reactive & Functional Polymers 74 (2014) 37–45

1 M aqueous solution of sodium chloride. The suspension was stirred (100 rpm) for 12 h before being centrifuged (4000 rpm, 10 min). The supernatant was removed and the clay retreated the same way three other times with fresh molar NaCl solution. The excess of NaCl was removed by stirring in water (400 mL, 100 rpm, 5 min) and centrifugation (4000 rpm, 10 min), followed by dialysis. The dialysis was performed on 2 g samples suspended in 200 mL of distilled water placed in a dialysis membrane (cellulose, 20 cm long, 20.4 mm diameter, MWCO 6–8 kD) against 1 L of distilled water. The distilled water was changed until free of chloride ions using the silver nitrate test (1 M AgNO3). After over-night standing, the suspension was decanted, left to evaporate and dried at 50 °C to afford the purified and sodium exchanged clay. Composition and analyses by XRD (2h = 0–60°) and FTIR, as previously described, are presented in Table 2 and Figs. 8 and 9 (Section 3.3).

Table 1 DSC studies of synthesized the polymers A, 3a–e and 4a–e. Texo (°C)

Td (°C)

DHd (J g1)

3a 3b 3c 3d 3e 4a 4b 4c 4d 4e A

– – – – – – – – – – 244

407 410 411 407 407 411 414 413 414 412 407

378.18 637.48 286.39 181.56 217.40 342.46 184.92 419.41 356.61 183.37 335.07

For structures, see Scheme 4.

4 Weight loss = 0.681mg % = 6.34

2

Heat Flow ( µv)

2.5. Metal ions extraction method

0

-2

DTA

0

-4

-6 -2

Weight Loss (mg)

EXO

a

Polymera

-8 TGA

-10

-4 0

100

200

300

400

500

Fig. 5. DTA–TGA curves for azidomethyl polystyrene (A).

Aqueous monometallic solutions of CdCl2H2O, Fe(NO3)39H2O, NiCl26H2O, CoCl26H2O and MgCl26H2O were prepared at a concentration of 50 mg L1 in relation with each metal ion in distilled water (pH 5.6). For the clays, a thin film was prepared prior to incubation directly into the flask by placing 0.18 g of clay in 20 mL of distilled water and leaving it to evaporate slowly at 50 °C. The polymer, or the clay (0.18 g), was incubated with 20 mL of the metal ion solution (1 mg) at 25 °C for 48 h. The suspension was filtrated on filter paper. The amount of remaining metal ions in solution was evaluated by atomic absorption spectroscopy (AAS) analysis of the filtrate after a 25-fold dilution in distilled water on a Perkin–Elmer AAnalyst 200 calibrated with solutions of the studied metal (0.5–5 mg L1). The results, average of three experiments, were expressed as percentages of extraction of the metal, based on its initial concentration (Figs. 9 and 10, Section 3.4).

95

699

1256

1605

3040

2933

90

706

2094

85

1240

3037

1607

(A) : R. T

2933

TRANSMITTANCE

(A) : T2

4000

3000

2000

1000 -1

Wave number (cm ) Fig. 6. ATR/FTIR of azidomethyl polystyrene (A) after the synthesis (R.T.) and after heating at 244 °C.

Fig. 7. Map of Tunisia and localization of the Jebal Es Sath and Jebal Sehib deposits near Gafsa.

Table 2 Chemical compositions of raw clays from Jebal Es Sath and Jebal Sehib.

a b

Elementsa

SiO2

Al2O3

Fe2O3

CaO

MgO

SO3

K2O

P2O5

Na

L.I.b

Raw Clay Jebal Es Sath

43.34

21.48

7.90

2.96

2.03

0.90

0.86

0.36

0.65

9.28

Raw Clay Jebal Sehib

39.52

19.87

15.06

7.87

2.87

0.90

0.915

0.23

0.35

9.28

Composition in wt.%. Loss on ignition.

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A. Ouerghui et al. / Reactive & Functional Polymers 74 (2014) 37–45 3,53Å

12,31Å 6,93Å

6,0x10

2

(d)

3,30Å 14,45Å

7,O1Å

3,51Å 4,39Å 4,18Å

Intensity

4,93Å

4,0x10

2

2,0x10

2

(c)

3,98Å

2,48Å

11,80Å

12,31Å

3,12Å

(b)

3,49Å 3,11Å 6,81Å 3,98Å

(a)

0,0 0

20

(2 )

40

60

Fig. 8. X-ray diffractograms of: (a) raw clay of Jeba Es Sath; (b) purified clay of Jebal Es Sath; (c) raw clay of Jebal Sehib; (d) purified clay of Jebal Sehib.

group was confirmed by infrared spectroscopy on which its strong characteristic absorption was seen at mN3 = 2094 cm1. Two sets of substituted alkynes, needed for the grafting onto the polymer, were prepared using usual procedures. The first set was composed of amides of propargylamine (Scheme 2). The amides 1a–d were synthesized using the corresponding acyl chlorides and coupling them on propargylamine in the presence of a base and a catalyst. The yields obtained were modest (66–79%) but the products were pure enough to be used without further purification. The structure of 1a–d was confirmed by characteristic signals in NMR (ca. d = 2.25 (triplet, „CAH) ppm in 1H and 70, 80 (C„C), 170 (C@O) ppm in 13C) and IR spectroscopies (ca. 3300 (m„CAH), 2100 (mC„C), 1640 (mC@O) cm1). The second set was also constituted of amides, but prepared from some anilines and propiolic acid (Scheme 3). The coupling was done using a classical dehydrating agent (DCC) and afforded the amides 2a–d are obtained with yields around 70% calculated after purification. Once again, the structures were in agreement with NMR (ca. d = 3–4 (singlet, „CAH) ppm in 1H and 77, 80 (C„C), 150 (C@O) ppm in 13C) and IR (ca. 3300 (m„CAH), 2100 (mC„C), 1640 (mC@O) cm1) analyses.

(h)

3.2. Synthesis and ATR/FTIR, DSC and DTA–TGA analyses of polymersupported triazoles (3a–e, 4a–e)

(g)

(f) 800 694 920 1034

0

4000

535

1442

40

1644

(e) 3641

Transmittance

80

3500

3000

2500

2000

1500

1000

500

-1

(cm ) Fig. 9. ATR/FT-IR spectra of: (e) raw clay of Jebal Es Sath; (f) purified clay of Jebal Es Sath; (g) raw clay of Jebal Sehib; (h) purified clay of Jebal Sehib.

Fig. 10. Percentages of metal cations extraction for the polymers 3a–e, 4a–e and A.

3. Results and discussion 3.1. Synthesis and characterization of azidomethyl polystyrene (A) and substituted alkynes (1a–d, 2a–d) Azidomethyl polystyrene (A), now commercially available, was prepared following the method of Mioskowski et al. from Merrifield’s polymer treated with sodium azide (Scheme 1) [44]. The needed azide bearing polymer (A) isolated from this reaction had a final loading of 1.96 mmol N3 g1. The presence of the azido

The alkynes (1a–d and 2a–d) and azidomethyl polystyrene (A) were then reacted together according to a CuAAC procedure in order to form polymer-supported 1,2,3-triazoles with different substituents in position 4 (Scheme 4). Azidomethyl polystyrene (A) was treated with an excess of the alkyne in the presence of a base (DIPEA) and cuprous iodide as a catalyst, in order to assure a complete transformation with a good kinetics. In order to have some insight onto the groups borne by the triazole ring on the polymer, we completed the first alkyne series by using phenylacetylene (1e) and the second one using methyl propiolate (2e). The reactions were completed in ca. 24 h or less, the reactions having been followed by ATR/FTIR and considered as complete when the vibration of the azido group has disappeared (mN3 = 2094 cm1). The yields in polymers, in relation to the reaction itself, are difficult to evaluate by mass increase due to the small quantities used and loss during transfers. Nevertheless, the reactions were considered as quantitative based on the ATR/FTIR analysis. The resulting polymers (3a–e and 4a–e) did show a significant decrease of the vibration band of the azido group of azidomethyl polystyrene (A), and no vibrations from the starting alkynes (C„C, „CAH) can bee seen on their IR spectra as shown in Figs. 2 and 3. Characteristic vibrations of amide bonds, i.e. NH, C@O, except for 3e and 4e (for 4e an ester mC@O instead), and additional vibrations from the substituted triazole and some of its substituents can be seen on the spectra by comparison with the spectrum of the starting polymer A. DSC analysis of the polymers gave similar curves in Fig. 4 and Table 1. Initial transitions (Tg, Tc, Tm) were difficult to see and only an endothermic event was observed for all polymers around 410 °C. This endothermic transformation has been attributed to the decomposition of the polymers. In the case of azidomethyl polystyrene (A), thermal analysis was studied by coupled DTA and TGA (Fig. 5). Results shown that the exothermic peak at 244 °C corresponded to a weight loss (Dm = 0.681 mg, 6.34%), this had been attributed to the decomposition of the azido group onto the polymeric structure [44]. ATR/FTIR spectra of A at different temperatures shown that the azide vibration (2094 cm1) disappeared from the spectra over 244 °C (Fig. 6), The increase of temperature from 25 at 244 °C was realized during 60 min at a speed of 5 °C min1.

A. Ouerghui et al. / Reactive & Functional Polymers 74 (2014) 37–45

3.3. Natural clays: harvesting and composition The clays were collected in two well-known Tunisian deposits in the Gafsa area: Jebal Es Sath (between Gafsa and Metlaoui, around 34°230 N/8°310 E) and Jebal Sehib (near M’dhilla, around 34°120 N/8°420 E); as shown as Fig. 7. The composition of the raw clays was quite similar but Jebal Es Sath raw clay contained half the amount of iron (7.90%) than the Jebal Sehib one (15.06%). Furthermore, Jebal Es Sath contained less calcium (2.96% vs 7.87%), and was more rich in sodium (0.65% vs 0.35%), than the Jebal Sehib ferruginous and calcium-rich clay (Table 2). Treatments to obtain purified sodium-exchanged clays were followed by XRD (Fig. 8) as well as by FTIR (Fig. 9). XRD spectra of raw Jebal Es Sath (Fig. 8a) showed typical rays of a sodium-rich smectite clay (12.31 Å and 3.98 Å), probably a Montmorillonite [(Na,Ca)0,3(Al,Fe,Mg)2Si4O10(OH)2nH20], containing kaolinite (Al2Si2O5(OH)4; 7.01 Å and 3.51 Å). After purification of Jebal Es Sath (Fig. 8b), the XRD spectra recorded was quite similar but characteristic for a sodium smectite (11.80 Å and 3.98 Å). In the case of raw Jebal Sehib clay (Fig. 8c), the XRD was typical of a calcium-rich smectite (Montmorillonite, 14.45 Å) together with kaolinite (7.01 Å and 3.518 Å), quartz (SiO2; 4.18 Å and 3.30 Å) and other impurities. Once purification conducted on Jebal Sehib clay, the XRD spectra (Fig. 8d) was the one of a sodium-rich smectite clay (12.31 Å and 3.98 Å) still containing some kaolinite (7.01 Å and 3.51 Å). In ATR/FTIR spectra of studied clays (Fig. 9), the bands observed at 3641 cm1 is attributed to the Al–Al–OH vibration [45], and those observed at 1644 cm1 correspond to OH frequency for the water molecule adsorbed on the clay surface, as well as those for SiO at 1034 and 920 cm1. Spectra of raw clays (Fig. 9e and g) displayed bands relative to quartz (694 and 535 cm1) and calcite (1442, 800 cm1); which are absent of the clays (Fig. 9f and h) after the purification. These results, as well as DRX analyses, confirmed the purification and sodium-exchange of the clays [45,46]. 3.4. Metal ions extraction by the polymer-supported triazoles and raw and purified clays When looking at metal extraction results obtained with the modified polymers, the first observation was that most polymers do not seem very selective for the studied metal cations, but exceptions, and that they had extraction efficiencies around or below 40% (total average 35.08 ± 1.36%, Fig. 10). For magnesium (II), the total average extraction efficiency is 39.16 ± 0.47%, the series 3a–e (35.86 ± 0.59%) and 4a–e (42.46 ± 0.34%) being equivalent. Only polymers 4b (44.82 ± 0.49%), 4c (43.17 ± 0.07%) and 4e (49.50 ± 0.74%) were a little more efficient for Mg2+. In the case of iron (III), the average is 34.64 ± 1.81% (3a–e = 37.01 ± 1.69%, 4a–e = 32.27 ± 1.92%). The best extraction in the two series was polymer 3a, which reached 48.59 ± 4.29% for Fe3+. Cobalt (II) was not very efficiently removed for the solutions by any of the polymers, with a global average of 18.29 ± 1.18% (3a–e = 16.99 ± 1.13%, 4a–e = 19.58 ± 1.23%). Only the resin 4c was better with 31.55 ± 2.69% removal of Co2+. With nickel (II), none of the polymers was over the average of 38.03 ± 1.63% extraction efficiency for Ni2+ (3a–e = 39.40 ± 1.75%, 4a–e = 36.66 ± 1.52%). In the case of cadmium (II), the average removal of this metal was a little higher than for others, reaching 45.28 ± 1.69% (3a–e = 52.79 ± 1.30%%, 4a–e = 44.56 ± 1.97%). The best polymers were found to be 3b (51.03 ± 0.41%), 4a (48.58 ± 1.84%) and 4c (48.96 ± 2.12%); with the best one being 3a (97.75 ± 0.80%). The starting polymeric azide A was also tested as a blank even if no azide was left in the synthetic polymers after the CuAAC reaction. The polymer A was not very good at removing Mg2+

43

(2.00 ± 0.03%), Ni2+ (3.75 ± 0.78%) and Co2+ (4.93 ± 3.21%); a little better for Cd2+ (11.31 ± 2.23%) and better at chelating iron (III) (60.77 ± 2.43%) than other metals. In summary, for the polymeric triazoles 3a–e and 4a–e, average extraction yields were in the following order: Cd2+ > Mg2+ P Ni2+ > Fe3+ >> Co2+; almost following the reverse order of ionic radii of the metal cations. During the reflexion on the design of our polymers, we were interested to probe the possible chelations when incorporating an amide function on the formed triazole. Most of the polymers found in the literature are bearing appendages of amine or pyridine nature (‘‘pendant’’ or ‘‘integrated design’’) or triazoles alone (‘‘triazole design’’) to chelate a metal ion prior to reduction inside dendrimeric structures, as an example. They were mostly used for catalytic purposes [47–51]. For the evaluation of our supported pendant amide triazole, we first studied the polymers 3a–e series. The aminomethyl substituent onto the triazole ring was substituted with increasing alkyl chain, as well as a phenyl ring, on the amide carbonyl, in order to study steric hindrance and/or hydrophobicity influence. In this series, polymer 3e was synthesized as a blank, since no amide function was present. When looking at the results, all polymers 3a–d gave extraction efficiencies similar to 3e. We make the hypothesis that only the triazole nucleus (‘‘triazole design’’) maybe implicated in the chelation since amide substitution seems not to have an influence. Furthermore, proximal sites onto the polymer do not seem to participate in a multidentate fashion. The only exception and selectivity found is with Cd2+ and polymer 3a (98.7 ± 0.8%). It is quite difficult to draw a clear picture of any of the results when looking at an interfacial process. We found out that the number of chelating entities for each metal was quite high. As an example, for polymer 3a and cadmium, 0.30 mmol (0.18 g polymer, 34 eq.) of triazole was in contact with 0.0089 mmol (1 mg) of cadmium. Extraction efficiency was 97.7 ± 0.8%, giving 0.0087 mmol cadmium for 0.30 mmol triazole, or 35 triazoles for 1 cadmium. The chelation mode is not evident since the chelating entites were already in excess. As added proofs, selectivity and good extraction levels for Cd2+ were observed with poly[N-isopropylacrylamide] crosslinked with tetrakis(1-vinyl-4triazolylmethyl)ethylenedia-mine [52]. Bis-chelation in a ‘‘triazole’’ or ‘‘integrated designs’’ was also postulated in the case of a triazole substituted calix[4]arene [53]. In the second series (4a–e), we were interested in the possibility of multisite complexation by oxo functions on substituted aryl rings to mimic crown ethers chelation type. For this purpose, polymers substituted by a simple phenyl ring (4a, no oxo functions), dimethoxy (4b), ethylenedioxy (4c) and crown ether (4d) bearing phenyl groups were compared to 4e as a blank (methyl ester, no amide or aryl groups) [54]. Unfortunately, our starting hypothesis did not give the awaited results. All polymers gave similar extraction performances as 3a–e for all metal ions in this study, except 3a (97.75 ± 0.80%) for Cd2+. For the shake of comparison with natural materials, the evaluation of Tunisian clays was conducted with the same metal ions (Fig. 11). For the Jebal Es Sath samples (JES raw and purified), the extraction of Fe3+, Co2+ and Ni2+ were almost equal and in the range of 92.20 ± 0.27% to 95.94 ± 2.23%. Mg2+ was poorly extracted by raw JES (2.69 ± 0.25%) but more by purified JES (18.47 ± 0.05%). For Cd2+, the extraction was at an average level for both raw (63.98 ± 1.48%) and purified JES (55.46 ± 3.20%). In the case of Jebel Sehib deposit, the same levels of extractions were reached for Co2+ and Ni2+ (93.81 ± 0.26% to 96.74 ± 0.17%), but were higher in the case of Fe3+, with 99.50 ± 0.16% and 84.76 ± 2.21% for the raw and purified JS respectively. Extraction of Mg2+ ions was similar to JES with 3.64 ± 0.13% (raw JS) and 25.17 ± 0.74% (purified JS) efficiencies. The extraction of Cd2+ was better with these samples, reaching 84.76 ± 2.21% (raw

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A. Ouerghui et al. / Reactive & Functional Polymers 74 (2014) 37–45

when compared to the synthetic polymers and was found to be Fe3+ P Co2+ P Ni2+ > Cd2+ >> Mg2+. These initial results, and the very good efficiency of 3a for cadmium (II), prompt us to continue the study of polymer modifications using click and other approaches in order to devise new extractants. Further studies of their application in real situations will be performed and be reported in due course as well. Acknowledgments This project is financed by Tunisian 05/UR/12-05 and French UMR 8151 CNRS/U1022 INSERM Grants. A.O. is grateful to EGIDE International Program fellowship. Fig. 11. Percentages of metal cations extraction for the raw (Raw) and purified (Pur.) Jebal Es Sath (JES) and Jebel Sehib (JS).

References JS) and 64.70 ± 4.95% (purified JS). The average extraction efficiencies followed the order: Fe3+ P Co2+ P Ni2+ > Cd2+ >> Mg2+; almost exactly identical to the order of the dimensions of the metal cations. Furthermore, the selectivity was the exact opposite than the polymer’s one. All the clays gave better extraction yields than the polymers (around 72% vs 35%), but as them, they were not very selective for Fe3+, Co2+ and Ni2+. Only the less ferruginous Jebel Sehib samples seem to extract a little more iron (III). For magnesium (II), the clays did not accommodate a lot of this ion; thus being selective for its exclusion. Extractions reached only 18.47 ± 0.05% to 25.17 ± 0.74% for purified samples, and 2.69 ± 0.25% to 3.64 ± 0.13% for the raw ones. For cadmium (II), percentages of extraction were better than for Mg2+. Differences were however observed between the ferruginous Jebal Es Sath samples and the Jebel Sehib one in the case of cadmium (II) extraction. Purified and raw JES extracted 55.46 ± 3.20% and 63.98 ± 1.48% of Cd2+, and purified and raw JS reached 64.70 ± 4.95% and 84.76 ± 2.21% extraction yield respectively. The results obtained with those clays can be compared with some published in the literature but, of course, the studied clays were not issued from the same deposits, and do not exhibit the same composition or structure. For Ni2+, absorption was usually the best and near a 100% efficiency [21]. In the case of Co2+, the extraction is often better than Cd2+ and near the Ni2+ one [55]. For Cd2+, similar moderate to good extraction efficiencies were observed for a smectite, reaching averages of 67% for the raw clay and 80% for the purified one [56].

4. Conclusions In this work, we first made chemical grafting of chelating unit onto poly[styrene] using CuAAC procedure between the azided polymer and selected alkynes. The synthesized polymers were then characterized conducting FTIR and DSC studies. They were then evaluated for their ability to extract metallic ions from solution; namely Mg2+, Fe3+, Co2+, Ni2+ and Cd2+. The polymers were found to extract these ion at low level (ca. 35%) without a clear selectivity, but seemed to have an affinity following the order Cd2+ > Mg2+ P Ni2+ > Fe3+ >> Co2+. However, the polymer 3a was very selective and efficient for Cd2+ extraction (97.75 ± 0.80%). These results were compared with two natural clays from SouthWest Tunisia, which had been harvested in the Gafsa area at Jebal Es Sath and Jebel Sehib (raw and purified). The clays had better extraction efficiencies, around 96%, for Fe3+, Co2+ and Ni2+. They were selective since they did not remove more than 25.17 ± 0.74% of the Mg2+ ions. For Cd2+, it was removed with an average of 60% for the Jebal Es Sath samples and of 75% for the Jebel Sehib ones. In the case of the clays, selectivity order was reversed

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